[gumby79]

If the Grapevine(I-5) is too much grade, then Donner(I-80 ) is out of the question. Donner is the big brother to the little Grapevine ,mom dusen't have to know.
1+ on US 95-US395 droping over the Cascade mount range into Eugene, or . Skip the Siskiyou mountain range on I-5.

Susanville to Klamath Falls
Options
Susanville,CA, CA 139N changes to OR 39 at state line to Klamath Falls, OR, was a 50+mi shortcut with a steep tight climb up out of Susanville . This was my routine commute Klamath Falls to Reno once a week 600 mi round trip This is a length restricted route 35' king pin to rear axel. by Susanville you will have to deside just how comfortable you are with the trailer and supper long wheelbase truck.
The other option is stay on US 395 to Alturas, west on CA299 ,noth on CA139/ OR39 to k-falls.

MY personal perspective avoid northern I-5.Shasta to Eugene . The Terrain its bad enough that both BNSF and UP Railroads rerouted off the I-5 corridor in Dunsmuir, CA through Klamath Falls, OR adding over 100mi but cuts fuel use witch cost more than the extra 100+ mi of track maintenance in steem days . I -5 was going to go through K-falls but getting 4 Lane interstate, fright/ passenger rail, and frontage road past the lake was too expensive . There are still some deadly narrow spots with only single track and a 2 lane road with no sholders on eather side.